The physiology of grass production under grazing. I. Characteristics of leaf and canopy photosynthesis of continuously-grazed swards.
Perennial ryegrass swards were continuously grazed by sheep. One sward was grazed to maintain an LAI (lamina) close to 1.0 ('hard' grazed) and a 2nd sward an LAI close to 3.0 ('leniently' grazed). In the hard-grazed sward, the growing leaves were efficient in photosynthesis and, together with the youngest fully expanded leaves, contributed 77% to the net photosynthesis of the sward canopy. Sheaths contributed less than 5%, even though the area of sheath was almost as great as the area of young laminae. Despite the high photosynthetic efficiency/unit leaf area in the hard-grazed sward, canopy photosynthesis in this sward was substantially less than in the leniently-grazed sward. Calculated over the grazing season, April to Sept., the amount of gross photosynthetic uptake accumulated under lenient grazing was only marginally less than under infrequent cutting. The consequence to yield of the difference in photosynthetic uptake between the hard- and the leniently-grazed swards is discussed.